Social polymorphism in the sweat bee Lasioglossum (Evylaeus) baleicum (Cockerell) (Hymenoptera, Halictidae) in Hokkaido, northern Japan

Abstract

The life cycle and social behaviour of the sweat bee Lasioglossum (Evylaeus) baleicum (Cockerell) was investigated in two geographically separate populations in Hokkaido, northern Japan. Colonies were excavated throughout the brood rearing season from an aggregation in Nishioka forest park, Sapporo, and near Kawakita in eastern Hokkaido during 2000 and 2001. The Nishioka population produced two discrete broods during the year and was weakly eusocial; 57 % of workers were mated and 28 % exhibited some ovarian development, 12 -16 % of the first brood was male, and workers were on average 4.5 % smaller than their respective queen. In contrast, the population at Kawakita was solitary, and produced a single brood per year with an unbiased sex ratio. In addition however, there were some solitary colonies in the Nishioka population and evidence of a partial second brood in some colonies at Kawakita, suggesting differences between the populations are not fixed and that this species is truly socially polymorphic. L. (E.) baleicum is a member of the fulvicorne species group, which includes other examples of social polymorphic species as well as solitary and eusocial species, though this is the only species of this group so far known to exhibit a solitary/non-delayed eusocial polymorphism. Recent studies suggest that social polymorphism has both genetic and environmental influences, raising questions as to the relative import of each.

A halictid bee with sympatric solitary and eusocial nests offers evidence for Hamilton's rule

“…Therefore, this situation would enable us to test Hamilton's rule because the rule is satisfied only when the solitary option is disadvantageous. In L. baleicum, males have been produced in the F1 generation 17 ; however, only a part of the F1 females was inseminated in the study areas. The low insemination rate of the F1 generation is a normal condition in this species 17 .…”

“…In L. baleicum, males have been produced in the F1 generation 17 ; however, only a part of the F1 females was inseminated in the study areas. The low insemination rate of the F1 generation is a normal condition in this species 17 . Although the solitary F1 females in this study might not be inseminated (insemination status was not confirmable for these individuals), the sole F2 male that survived in their nest has matched genotypes with the nest owners.…”

“…After a short inactive period, a second brood is reared (F2) during mid-July to mid-August. No third reproduction has been observed most likely because the season is never long enough to complete a third reproduction 14,17 . The F2 females hibernate after mating and become new F0 foundresses in the following spring.…”

“…The F2 females hibernate after mating and become new F0 foundresses in the following spring. The F1 broods contain males and females, and a fraction of the F1 females are inseminated 17 . During the second reproductive period, many nests contain multiple adult females; however, some nests are maintained by a single female 15 .…”

“…For example, L. baleicum populations inhabiting cold areas rear only a single brood 14,17 , suggesting that early diapause is an optional strategy for F1 females. In addition, adult females occasionally intrude into an unrelated nest 14,15,19 .…”